1. Technical Field
The present invention relates generally to a projection device, and more particularly to an adjusting method of an iris of the projection device.
2. Description of Related Art
As shown in FIG. 5, a conventional projector 10 includes a light source 100, a parallel lens array 102, a P-S converter 104, an iris 106, a pair of spectroscopes 108, 110, three reflecting mirrors 112, 114, 116, five condensing lenses 118, 120, 122, 124, 126, three liquid crystal panels (LCDs) 128, 130, 132, a combination prism 134, and a projection lens 136. The light source 100 is used for emitting white light beams. The white light beams pass through the parallel lens array 102 to the P-S converter 104. The P-S converter 104 is used to convert P-polarized beam to S-polarized beam. Thus an extinction ratio of the light beams which equal to Rs/Rp increases. The light beam output from the P-S converter 104 is separated into color beams R, G and B of red, green and blue through the processing of the spectroscopes 108, 110. The color beams R, G and B are reflected at a right angle by the reflecting mirrors 112, 114, 116, respectively. The condensing lenses 118, 120, 122, 124, 126 are used for collimating the color beams R, G and B. Then the color beams R, G and B come into the LCDs 128, 130, 132, respectively, with image information corresponding to video signals. The combination prism 134 synthesizes the color beams R, G and B, and finally the projection lens 136 magnifies and projects the synthesized color beams onto a screen to display images.
FIG. 6 is a graph indicating a relationship of an extinction ratio and an angle of incidence of light of an LCD. As the angle of incidence of light increases, the extinction ratio decreases. FIGS. 7-8 show two different situations of the projector. The difference between the two situations is that the iris 106 of FIG. 7 is more open than the iris 106′ of FIG. 8. Thus only part of the beams can move through the iris 106′ of FIG. 8. Part of the light is wasted, and a luminous energy of the light decreases. Thus, the brightness of images of the projector of FIG. 8 decreases. Because the iris 106 of FIG. 7 is more open than the iris 106′ of FIG. 8, an angle α of incidence of light of FIG. 7 is larger than the angle β of incidence of light of FIG. 8, and the contrast of images of the projector of FIG. 7 is less than that of FIG. 8. Thus the conventional projector cannot achieve a projection image with high contrast and high brightness at the same time.
Therefore, a new projection device is desired to overcome the above describe shortcomings.